Building 1:1 previews will (always, I think) re-build standard previews too, even if they would not be built by the 'Build Standard Previews' function. So, if the "discard 1:1's" feature were not broken in Lr5, it would be a way to recreate standard previews with the new settings (i.e. discard and rebuild 1:1's). - good idea Bob ;-).

PS - I tested the hypothesis by using Lr4, and am extrapolating to Lr5. Here is what I did:

1. Created a 1 photo catalog.
2. Built 1:1 previews with large standard previews.
3. Checked steady-state preview folder size (you have to wait a while for Lr to consolidate files and database entries...).
4. Discarded 1:1 previews, changed standard settings to small, and rebuilt 1:1 previews.
5. Again, after waiting for steady-state to be reached, the folder size of previews was notably smaller.

In my case (for regular previews), all (4) cpus are going, but are only utilized at 100% for part of the time, so total average cpu activity is like 2/3 (67%) - about like an export... - i.e. 3-6 seconds per photo for 12MP raws (D300), depending on editing.

Granted cpu utilization for smart previews is closer to 95%, and take about 2/3 of a second, so yeah - a lot faster. Still, *way* more needs to be done to render regular (baked) previews than to create smart (unbaked) previews.

PS - I have no idea whatsoever why Lr can't render regular previews using 95% cpu too, but even so that would make it 2-4 seconds instead, but not .7 seconds like smarts.

All 12 are going (6x2), but the average use is only about 20% and it takes about 5 secs to render a D800 nef. With the smart previews, all 12 hit max for quite long periods; I haven't seen my cpu get so hot!

Sounds normal. There is obviously something going on that prevents max cpu utilization but I dunno what the bottleneck is. I've heard claim that it's on purpose, so Lr UI operates smoothly whilst rendering, but I never bought that for a second.

It took a long time for the resolution war to reach DSLRs, but I bet a lot of folks (e.g. Lr users) long for the good ol' days when DSLR manufacturers were striving for fewer but better pixels ;-}.

I think, unless you are shooting with a very sharp lens, with flawless technique, and at low ISO, a lot of those hi-rez pixels are wasted.

But conversely, if you are shooting with a very sharp lens, flawless technique, and low ISO, a lower rez camera may not be taking full advantage of your lens sharpness.

Maybe worth noting: Much of Lr (can be roughly thought of as the VC part of MVC, if that means anything to ya), is written in Lua, and taps into dev code of ACR, written originally for Ps, for rendering. The integration is looser than optimal for Lr. Not sure what that means exactly, except it's different than most raw softwares where all pieces were written together... But when rendering smart previews, ACR is not in the loop.

Summary:
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Although it seems standard previews should be re-built if standard preview building is invoked manually, after standard preview settings have changed, just fixing the "discard 1:1 previews" feature would provide an adequate work-around, i.e. to rebuild standard previews: